This invention relates to electrolytic capacitors and more particularly, to a novel electrolyte comprising salts of heteropolyacids wherein the hetero atom is silicon, of the group consisting of silicotungstic acid and silicomolybdic acid.
The prior patent to Hand, U.S. Pat. No. 3,502,947, describes a capacitor electrolyte consisting of a heteropolyacid such as phosphotungstic acid, in a suitable organic solvent, such as N-N'-dimethyl formamide, neutralized with ammonia to a pH of 7.7. Greater stability is alleged over wide temperature ranges, long shelf life and a low current leakage.
In the present invention, the use of heteropoly compounds wherein silicon is the heteropoly atom results in electrolytes having greater reliability at high temperature operation, 125.degree. to 150.degree. C. for example. Additionally, the electrolytes of the present invention result in capacitors having superior low temperature properties as compared to conventional electrolyte solutes, such as salts of organic acids, borates or borate complexes.
It is an object of this invention to provide a capacitor having long shelf life, superior reliability at high temperature operation and superior low temperature properties, the said capacitor having an electrolyte comprising a silicotungstate or silicomolybdate salt in an organic solvent.
A further object of the invention is to provide capacitor electrolyte as set forth above wherein the salt consists of ammonium, alkali metal, e.g. sodium, quaternary ammonium, or amine salts of the heteropolyacid, and the solvent consists of polar organic solvents.
Further objects will become apparent from the following specification and claims.
The present invention contemplates the use of a novel electrolyte in a capacitor of conventional construction. Such capacitor employs an aluminum foil anode and an aluminum foil cathode, separated by a paper spacer. After being wound into a cylindrical shape the foils and separator together with the electrolyte are sealed in a suitable container.
The novel electrolyte of this invention consists essentially of a salt of silicotungstic acid (H.sub.4 SiW.sub.12 O.sub.40) or silicomolybdic acid (H.sub.4 SiMo.sub.12 O.sub.40) in a polar organic solvent. The formulas set forth in the preceeding sentence give the anhydrous compositions of the acids and are written so as to reflect the structure of the heteropoly anion which consists of a central Si atom surrounded by a W.sub.12 O.sub.40 (or Mo.sub.12 O.sub.40) cage. The cage is made up of linked WO.sub.6 (or MoO.sub.6) octahedra sharing oxygen atoms.
Commercial silicotungstic acid is sold under the formula SiO.sub.2.sup.. 12WO.sub.3.sup.. 26H.sub.2 O. This has the same stoichiometry as H.sub.4 SiW.sub.12 O.sub. 40.sup.. 24H.sub.2 O, which is a hydrated form of silicotungstic acid. Although heating could reduce the number of associated water molecules to less than 24, this would not affect the chemical behavior because the functional part of the molecule would be unchanged.
Similarly, sodium silicomolybdate is available under the formula Na.sub.4 [SiMo.sub.12 O.sub.40 ]. x H.sub.2 O, when x is 10-15. For the purposes of this invention the actual value of x is unimportant since the water content can be lowered if necessary by drying, or water can be added to the final electrolyte if that should be desirable.
The description of the acids as H.sub.2 SiW.sub.12 O.sub.40 and H.sub.4 SiMo.sub.12 O.sub.40 herein is not intended to limit them to the anhydrous form, but is to be understood as including their respective hydrated forms as well. Also, the terms "silicotungstate" and "silicomolybdate" refer to anions [SiW.sub.12 O.sub.40 ].sup.-.sup.4 and [SiMo.sub.12 O.sub.40 ].sup.-.sup.4 respectively, regardless of the formulas which may be employed by chemical suppliers as a result of past tradition and practice.
As salts there may be employed the alkali metals, e.g. sodium, or ammonium quaternary ammonium or amine salts, either prepared in situ in the solution, or added to the solution as such. The solvent may be any of the commonly employed polar organic solvents employed in electrolytic capacitors. Examples of suitable solvents which may be employed are dimethylformamide, N-methylformamide, butyrolactone, N-methylpyrrolidone, dimethylsulfoxide, ethylene cyanohydrin, ethylene glycol, monomethyl ether of ethylene glycol and monoethyl ether of ethylene glycol.
The invention will be further illustrated in the following examples: